1. Field of the Invention
This invention pertains generally to the field of aircraft radio altimeter systems.
2. Description of the Related Art
Radio altimeter systems often employ transceivers from which a radio frequency (“RF”) signal may be transmitted continuously towards the ground in a beam at an assigned frequency and received in a return RF signal comprised of a plurality of reflections having different frequencies. Because a direct relationship exists between frequency and height above the ground, the height of an aircraft above the ground may be determined by analyzing the frequencies of the reflections. This analysis may be accomplished by subjecting the digital counterpart of the returned RF signal to a frequency spectrum analyzer having a relatively wide frequency range bracketed by or defined with low and high bounds. After the analyzer has performed an analysis, data representative of radio altitude (“RA”) may be generated and provided as source data to one or more user units. Although the analysis is performed within a wide frequency range, the frequency range due to the plurality of reflections is comparatively narrow during most flight operations.
When one aircraft overflies another aircraft and the overflown aircraft falls within the beam of the transmitted RF signal, the frequency range of the return RF signal may become comparatively wide because the plurality of reflections includes lower-frequency reflections caused by the overflown aircraft. When the digital counterpart of the returned RF signal having the wider frequency range is subjected to the same frequency spectrum analyzer, data representative of a lower RA—but not of the height above the ground—may be generated and provided as source data to one or more user units. Although an encounter with an overflown aircraft may be fleeting in nature, transient RA data not representative of the height above the ground could nevertheless be generated and subsequently provided as source data to one or more user units. If this transient RA data is provided to an indicating system, inconsistent RA information could be presented to the pilot. If it is provided to a terrain and awareness warning system, a false alert could be triggered and presented to the pilot. If it is provided to an automatic flight control system, automatic flight controls could be abruptly and unexpectedly disconnected.